Adrenocortical cells require cholesterol for steroid hormone synthesis. This precursor can be obtained from plasma lipoproteins or by endogenous synthesis. The proportion of the cholesterol utilized from these sources may depend on the availability of plasma liporoteins, the functional state of the cells, and the species of animal. Thus far, studies have concentrated on the whole adrenal. However, the different zonal cell types may also vary in their uptake: synthesis of cholesterol and be regulated differentially for this function. It has been suggested that steroid-secreting cells possessing abundant lipid droplets take up most of their cholesterol from the plasma (Fruhling et al, 1973), while those possessing abundant smooth-surfaced endoplasmic reticulum synthesize a large proportion of their own cholesterol. Neither of these hypotheses has been tested directly. The guinea pig adrenal has three distinct zonal cell types: zona fasciculata with abundant lipid droplets; zona reticularis with abundant smooth reticulum; zona glomerulosa with little lipid and a paucity of smooth reticulum. Thus the guinea pig adrenal provides an ideal test system for differential uptake vs synthesis among cortical cell types. It is the purpose of this proposal to use separated guinea pig adrenal zonal cell types to examine the ratio of cholesterol obtained from uptake vs endogenous synthesis for steroid biosynthesis. Peroxisomes of adrenocortical cells, small organelles associated with the smooth reticulum, contain a fatty acid Beta-oxidation systems separate from that in the mitochondria, which increases in activity in response to ACTH. The guinea pig adrenal provides an excellent tissue in which to investigate the extent to which acetyl CoA generated by this enzyme system is utilized for de novo cholesterol synthesis.